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SKINNER POWER SYSTEMS L.L.C.8214 EDINBORO ROADERIE, PA 16509 USAPhone: 814-868-8500Fax: 814-868-5299email: jeff@Skinnerpowersystems.netweb: www.skinnerpowersystems.net

Turbine Generator Sets 101Skinner Power Systems LLC

September 2006Rev 1 May 2009

Basic turbine generator set information

The most basic, easiest form of energy savings is to recoup electricity costs by burningsomething. This can be a fuel byproduct of the item being made (example, wood wastefrom a furniture maker), waste products (oil from auto or truck fleets), biomass products(sugar cane bagass or cornstocks), or even more exotic burnable things such as glycerinfrom biodiesel plants or flammable gases from petrochemical operations.

Energy costs are also decreased by utilizing steam turbines in place of pressure reducingstations in high pressure steam systems where plant process equipment requires lower  pressure steam. Many plant processes require heat and/or steam, and in installationswhere high pressure steam is made (150 psig and up), and the process requires lower  pressure steam (say 15 or 20 psig), a steam turbine can be installed in place of a pressurereducing station. The turbine acts the same as the pressure reducing device, and theenergy removed from the steam is converted to mechanical energy in the turbine, whichcan then drive a generator.

Other energy costs saving similar to the manufacturing plant application can be seen inlarge institutional buildings where steam is used for heat, cooking, or air conditioning.High pressure steam can be distributed to a college campus for example, and at key points steam turbine generator sets can be installed as pressure reducing devices, and electricity generated for use on the campus. Eighty years ago, virtually all hospitalsutilized the old reciprocating steam engines in their powerhouses to do exactly this typeof energy production. (Back then, the concern was to simply make reliable power yourself, rather than save utility costs.)

A summary of this can be explained this way: The “fuel” was paid for once (lumber for the furniture maker, engine oil for the truck fleet, or gas or coal for the college heatingsystem). The savings is in the “free” fuel, in that the waste lumber was already boughtand paid for to originally make furniture, but the residual waste can be used in the boiler;the oil was purchased as a lubricant and is now used in the waste oil burner; and the gasor coal was purchased primarily for heating the school and the turbine replacing thereducing station is using “free” steam. It’s a matter of what you pay for the fuel costs and why and how you look at the application.

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This should not be confused with the most efficient way to produce power, irregardless of cost. From an engineering standpoint, and practicality of the system, the easiest way and most efficient means of producing electricity is to use a standard combustion engine(diesel, gas, or natural gas). The energy in the fuel is more or less directly converted into

 power. Fuel in, power out, a little bit of heat as a byproduct. Since it is this simple, thetotal “package” is quite efficient.

With a system utilizing a steam boiler, there are energy losses in heating the steel in the boiler, the water, the piping and valves, the steam turbine, what to do with the exhaust or condensate (condenser equipment), the associated equipment costs (pumps, fans,foundations), operating considerations (chemical treatment of the water, ash residue, and air quality equipment), fuel source and handling, and labor costs to make these itemswork.

The limited type of fuels available to use in a combustion engine is its downfall. It’s

costly and it is limited to what is available in a burnable medium (natural gas or a good quality liquid such as diesel. The boiler on the other hand, can burn just about anything,and that is what the appealing factor is in the long run. No one has yet made a dieselengine capable of burning a piece of a wood pallet or corn stock.

General Steam Turbine Design

Standard Nomenclature

Steam Turbine: prime mover, which converts the thermal energy of steam directly intomechanical energy of rotation.

 Noncondensing Turbine: steam turbine designed to operate with an exhaust steam pressure equal to or greater than atmospheric pressure.

Condensing Turbine: steam turbine designed to operate with an exhaust steam pressure below atmospheric pressure.

Steam Turbine Stage: consists of a "matched set" of stationary nozzles and rotating blades. A pressure drop occurs in a steam turbine stage generatingkinetic energy which is converted to mechanical work.

Impulse Stage: consists of stationary expansion nozzle(s) discharging thehigh-velocity steam jets on the rotating blades. A pressure drop occursonly in the stationary nozzle(s). Impulse stages consist of two types:

Pressure Impulse or Rateau Stage: consists of stationary expansionnozzle(s) and one row of rotating blades.

Velocity-Compounded Impulse or Curtis Stage: consists of stationary

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Turbine Operating Conditions in a Single Stage Turbine

The heart and soul of any steam turbine generator installation is the steam conditions and  power output that must be matched together. In most cases where existing steam plantsare utilized, the inlet pressure is already determined by the boiler. Typically, most boilers

are rated at 250 psig, a fairly common number. Some applications can use steam pressures as low as 80 psig, while other more specialty applications may see pressures of 400, 600, or even 900 psig. The inlet pressure of the steam is the first factor in the termTURBINE OPERATING CONDITIONS.

The second part of these conditions is the temperature of the steam. This again is afunction of the boiler. For most applications, this will be the saturation point at theoperating pressure. For example, a 250 psig boiler has a steam temperature of 4060 F.At 300 psig operating pressure, the steam temperature is 4220 F.

Temperature can be increased in the steam by applying superheat to the boiler.

Essentially, this is simply creating a higher than normal steam temperature. By adding1000F superheat to 300 psig steam, the temperature would then be 522

0F.

The third consideration in turbine operating conditions is the exhaust pressure, the pressure at which the steam will leave the turbine. This is usually predetermined by thefacility. The majority of plants operate steam process equipment anywhere from 5 psigto as much as 175 psig. The norm is usually closer to 50 psig, more or less.

The more the plant needs heat in its processes, the more likely the exhaust pressure fromthe turbine will be higher rather than lower. For the most part, few plants require exhaust pressure above 100 psig, but certain applications do.

From a design point, and practicality of the turbine design, exhaust pressures are limited to about 175 psig no matter what, and, usually limited to certain size models. This is anengineering criterion, in that it would be very impractical to build a large single stageturbine with a very high backpressure. It would simply not work.

The fourth part of the turbine operating conditions equation is the speed the turbine needsto run at.

Depending where you live in the world, many turbines have one of these operatingspeeds: 1500 rpm, 1800 rpm, 2900 or 3000 rpm, and 3600 rpm. These numberscorrespond to standard electric motor speeds.

The exceptions are if the driven equipment needs to operate a different speed, or if thedriven equipment requires a gear reducer for the turbine to produce the horsepower or efficiency. Again, the rule of thumb is the faster the turbine rotates, the higher thehorsepower and the less steam it takes to make that horsepower. For example, in thesynchronous generator applications, 1500 or 1800 rpm generators are standard, are themost cost affective speed for generators, and cannot be engineered to operate at higher 

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speeds. The turbine may require a speed of 4000 rpm to not only be efficient, but mustrun at the high speed to make the horsepower.

Induction generator sets are either 1500/1800 rpm or 3000/3600 rpm. The difference ingenerator types will be explained shortly.

The turbine speed where the unit is direct coupled to the generator (or pump), will bedetermined by the driven equipment. If the pump needs 3600 rpm, the turbine will be built for 3600 rpm. In applications where a gear reducer is warranted, Skinner will pick the turbine speed for the most efficient use of the steam.

The fifth and last turbine operating condition that needs to be determined is horsepower.This again is a requirement of the driven equipment stating a horsepower rating, or whatamount of steam is available to convert into horsepower. For example, a generator rated at 500 Kwe will require 27603 #/hr of steam at 300 psig, 500

0F inlet temperature, 50 psig

exhaust, and a speed of 3830 rpm. If the plant boiler can only produce 15000 #/hr of 

steam at 300 psig, 500

0

F inlet temperature, 50 psig exhaust, and a speed of 3830 rpm, theturbine will only produce 367 HP or 260 Kwe.

The operating conditions a turbine must operate in and the amount of steam required tomeet the desired output is the balancing act the turbine manufacturer must perform. Theefficiency of how well we do accomplish this is what determines the size of the turbine,speed, controls, and encompasses the engineering package of any turbine. Some aresimple to determine, others may have many considerations.

The five basic operating conditions and how they eventually relate to the system is one of the primary aspects of determining how much the turbine will cost.

Recapping, the five turbine operating conditions are:

Inlet pressure in psigInlet temperature in degrees FExhaust pressure in psigSpeed in rpmHorsepower or Kw output

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Induction versus Synchronous Generator

In the most simplistic explanation of an induction generator, consider it nothing morethan a standard electric motor being driven by a steam turbine. If the motor is rated at3600 rpm, the turbine simply drives the motor a few rpm higher (3610 rpm for example),

and the motor now becomes a generator.

In operation, the turbine is started and run up to a speed of a few rpm less than 3600 rpm.The motor is actually put on line as a motor, which brings the speed of both to 3600 rpm.The motor is now drawing power from the plant electric system. The turbine speed isslowly brought up, and at just a few rpm above the 3600, the motor is now making power instead of consuming electricity. Think of this as the electricity flowing in the oppositedirection.

The advantage of an induction generator is it is simple to operate, simple to install,controls and safety devices are much less, and is a good application for peak shaving of 

the electric bill. The disadvantage of the induction generator is that is must be connected to the local utility grid (incoming power), to operate. The frequency synchronizing of theelectricity is accomplished by the external power source. If there is a utility outageoutside the plant, the motor will not operate and will not generate power. There must beoutside power for this to work, so any application of using an induction motor as a backup generator is not possible.

The synchronous generator is a true stand alone machine. You only need to synchronizewith the grid when you are sharing power. The synchronous generator requires noexternal matching of frequency, voltage, nothing. You can be your own island or independent utility if you want. Synchronous generators are a little more expensive, butthe real added costs are in the controls and safety devices. Since this type of generator istotally separate from the utility, safety devices are mandated by your local utility to prevent any electricity generated in your facility to be sent into their power grids duringoutages. This prevents the accidental electrocution of linemen who may be working on power lines in your area.

Up to about 250 kW, synchronous generators are available in 1800 and 3600 rpm speeds.Above 250 kW, and the only models available are 1800 and below. 3600 rpm generatorsare referred to as 2 pole machines, 1800 rpm generators are 4 pole machines. Poles refer the electric wiring internally on the generator. The higher the number of poles, the slower the speed of the generator.

Technical Explanation of Induction vs. Synchronous Generator Operation

A. Synchronous Generator 

Frequency control is applicable to the initial startup of the turbine-generator. The speed must be such that the frequency is correct, or the same as that of the electrical system(synchronized) before the generator is connected to the electrical system.

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 It is then applicable to an "operating" turbine-generator only when the electrical system isindependent of any other electrical system and a single turbine-generator is providing theelectrical power.

A NEMA Class D speed-governing system is generally considered appropriate evenfor stand-alone units.

Power generated control is applicable for turbine-generators operating in parallel with anelectric utility type electrical system, or one or more synchronous generators with anytype of driver.

The frequency is maintained by the utility. After the turbine-generator has beensynchronized with and connected to utility, the generator speed is fixed, i.e., the generator cannot rotate at a speed other than that corresponding to the frequency of the electricutility system.

The speed-governing system can then control the torque developed by the turbine only byvarying the power developed by the turbine. Closing or opening the governor valve willstill decrease or increase, respectively, the steam flow through the turbine and, therefore,the power developed.

The function of the speed changer is to permit changing the electrical power output of theturbine-generator. Adjusting the speed changer for a higher speed will result in thegovernor valve opening. The amount the governor valve opens is a function of the speed regulation characteristic of the speed-governing system.

When a turbine-generator is operated in parallel with another generator(s) withany type of driver, the electrical system frequency is normally controlled by only one of the units - the unit with the most precise speed-governing system. The other unit(s) willthen furnish electrical power as required through the adjustment of the speed changer(s).

A NEMA Class C speed-governing system is appropriate. It provides sufficientregulation to obtain proper positioning of the governor valve; and is accurateenough for ready synchronization.

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B. Induction Generator 

The speed-governing system must control the speed precisely above a valuecorresponding to the frequency of the electric utility system to which it is connected. The power generated is a function of the speed of the generator.

A NEMA Class D speed-governing system is recommended.

C. Standby Turbine-Generator 

Turbine-generators can be required for emergency or standby service in theevent of disruption of the normal electrical power supply. The Class of governor should  be a function of the electrical equipment to be supplied during a power outage and theresultant frequency control required.

A NEMA Class D speed-governing system should be considered.

Project Scope

The typical plant steam turbine generator system requires the minimum hardwarerequirements:

Steam Boiler Steam Boiler Accessories (stack, plumbing, foundations, monitoring equipment)Fuel SourceFuel Handling EquipmentSteam Turbine Generator SetElectrical Switchgear PackageCondenser or Makeup Tank if a closed loop system to return water to boiler PipingValves and Safety Devices on Piping

This is only a partial list of what may be required at any particular site. It would be aworthwhile endeavor to utilize local engineering firms for determining specific details.

The steam turbine generator set is actually one of the least expensive items in this list.By far, the boiler is the most expensive piece of hardware. The second most expensiveitem is the installation costs for this equipment.

What Is Required By Your Application

Every customer and potential customer who contacts us is either at the beginning of the process of considering installation of a steam turbine generator set (what do I need) or theend (what I want).

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To get to either point, we need to know the following:

Boiler Conditions – We do not manufacture boilers. A boiler company can tell you whatis possible from your fuel source, or can tell you what your present boiler can produce.The boiler manufacturer will need to address what accessory equipment needs to be in

 place (fuel handling equipment, stack requirements, etc).

If you presently have an existing boiler, this process is shortened considerably. The sizeof the turbine generator set is determined by what you have. We simply need to know the pressure (psig), temperature (F), and the exhaust pressure you need or require.

If you do not have a boiler presently, the process gets quite a bit longer. Based on your fuel source, the boiler maker will be able to provide you with sizing based on how muchfuel you have available. The BTU’s produced by the fuel will allow them to size the boiler, based on pressure, temperature, and flow. For example, if you have 8 tons of dayof burnable waste, the boiler can be sized at low pressure and temperature (150 psig)

which will produce a high flow of steam, or can be sized for a higher pressure (300 psig), but at a lower flow ( in pounds per hour). By working with the turbine builder, the bestoverall combination of pressure, temperature, and flow can achieve the best turbineselection for your application, taking into account turbine efficiency and turbine costs.

At the end of the boiler selection process, we need to know from you what pressure steam(psig), temperature of the steam, what the steam flow is (pounds per hour), and what your exhaust pressure requirement is. By working with a boiler designer and Skinner, we canachieve a practical boiler size for application with a steam turbine generator set.

Plant Electrical Characteristics: The kind of voltage (480V, 2400V, etc.), frequency (60Hz), phases (single or 3 phase), and any other information available.

Type of Generator: Induction or Synchronous: The key question is if there is a desire tooperate the turbine when the local utility is down. If you want to operate a turbinegenerator set during a utility outage, you must have a synchronous generator.

Electrical Switchgear Package: These are the controls that tie the turbine gen set into your  plant, taking into account your internal buss, the utility requirements for safety, and justwhat you want the controls to do. The switchgear can be simple or complicated. Wetraditionally avoid switchgear packages with our equipment, due to the localrequirements in your location. The switchgear is best left to your local electricalcontractor to supply. Since the controls on a diesel engine gen set are frequently thesame type used on steam turbines, a contractor familiar with diesel gen set installations(hospitals use diesels as emergency backup sources) will be of great help in your project.

A good general contractor or engineering firm can design the necessary piping for your installation. Again, local building codes will dictate many of the details you will need toaddress.

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The design firm can also be of great help in the overall plant steam system design. If yourequire a condenser, they can provide options on the most efficient type, location,interconnection, and operating details.

The general contractor will also address installation of the components.

What Does This Mean in Savings

The one issue you want to know is what is this going to save me in electricity costs.

A simple 100 kw turbine gen set does not seem that big, and you may not think of 100 kwin relation to a dollar figure. Analyze it this way:

If you were a large plant that had steam available to install a 100 kw generator set, and  produced 100 kw constantly for year, the avoided cost of purchased power (what youdidn’t need to buy) is:

100 kw (amount of power producer per hour) x 24 hours per day x 365 days =876,000 kw/hrs of electricity produced in one year X your current electric rate(for example: $ 0.12/kw-hr). That equals 876,000 x .012 = $ 87,600.00 savings per year.If the electric rate is less, your savings are less per year; if the rate is higher, it’s a greater savings.

A turbine has a typical lifespan of 40 to 50 years. Installation of a turbine generator setcan easily pay for itself in a few short years, in many times, less than 2 years if you havean existing steam plant.

This was an example of a small 100 kw gen set. If you have the steam capacity togenerate 1000 kw electrical savings could be in the neighborhood of a million dollars ayear.

Typically, in facilities where the capacity to make electricity is greater than the internalrequirement of the plant, excess electricity can be sold off to the utility. In mostcircumstances, the electric company will purchase electricity from your plant at aconsiderably lower cost than your current purchased price. The real savings is still inavoided costs, not the revenue from selling to your utility.

Summary

The task at hand for most potential turbine generator projects is getting a good handle on just what is necessary to make the system work as anticipated. A very good localcontractor can provide you with the various engineering requirements for your facility.Once we have the particulars on your steam conditions and utility requirements, we can provide you or them with recommendations on how best to proceed, and pricing to meetyour budget.

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